EVOLUTION OF THE TURFAN BASIN, CHINESE CENTRAL-ASIA

The Turfan Basin extends approximately 550 km east-west by 100 km north-south within northwest China and is located between branches of the intracontinental Tien Shan orogenic belt. At present it is an intermontane foreland basin loaded by southward transported thrust sheets at its northern margin. This deformation is a long-distance manifestation of the India-Asia collision, but the orientation and location of Cenozoic structures is strongly influenced by the pre-Cenozoic history of the region. The exposed basement marginal to the Turfan Basin consists of volcanics, granitoids, volcaniclastics, and deepwater sedimentary rocks associated with the North Tien Shan island arc and subduction-accretion complexes north of the North Tien Shan fault. This arc had accreted to the Central Tien Shan by late Carboniferous to early Permian times. The North Tien Shan fault represents a major boundary between Paleozoic arcs and accretionary complexes to the north and Proterozoic basement of the Central Tien Shan and Tarim Block to the south. Postcollisional thrusting appears to have provided a tectonic load for Lower Permian subsidence and sedimentation. Early postcollisional strata are turbidites, but there was a shoaling of facies until the deposition of Permo-Triassic conglomerates. No marine sediments have been deposited in the region since the Paleozoic. At least two Mesozoic episodes of compressional deformation are recorded at the northern margin of the basin, which was an intermontane foreland basin during this era. Lower Jurassic fluvial and lacustrine clastic rocks lie above a regional unconformity. An Oligocene unconformity marks the onset of coarse clastic sedimentation in the Cenozoic and probably indicates the start of major compressional deformation related to the India-Asia collision. Sedimentation rates have increased through the Cenozoic. The neotectonics of the Turfan Basin are dominated by a southward directed thrust system, emergent within the basin at the Fire Mountains range. Southward directed thrusting appears to have been a feature of Mesozoic age tectonics, and possibly the late Paleozoic tectonics of the basin; the most important neotectonic structures in the Turfan Basin follow older structural trends.